1. Field of the Invention
This invention relates to a process for purifying a nitrogen trifluoride atmosphere contaminated with a variety of components.
2. Description of the Prior Art
Nitrogen trifluoride is a colorless gas having a boiling point of about -129.degree. C. and a melting point of about -208.degree. C. Recently there has been interest in nitrogen trifluoride as a source of fluorine primarily for the preparation of fluorocarbons and fluoroolefins and as an oxidizer. Nitrogen trifluoride has advantages over elemental fluorine as a fluorine source in that it is relatively inert at low temperatures, e.g., 70.degree. F., whereas fluorine is not, and it can be compressed to high pressures, e.g., 1,000 psig for shipment.
Nitrogen trifluoride can be made by several methods. The primary methods include the electrolysis of molten ammonium acid fluoride; the reaction of fluorine azide with elemental fluorine and the reaction of ammonia and elemental fluorine. A particularly desirable process is shown is U.S. Pat. No. 4,091,081. In that patent nitrogen trifluoride is produced in good yield with reduced problems as compared to the prior art by reacting ammonia and fluorine in the presence of ammonium bifluoride.
In each of the above processes the nitrogen trifluoride atmosphere will contain contaminant materials, particularly nitrous oxide and fluorine compounds such as carbon tetrafluoride, dinitrogen difluoride and hydrogen fluoride.
Several U.S. Patents show techniques for producing nitrogen trifluoride and techniques for recovering the product. These include:
U.S. Pat. No. 3,043,662, which shows preparing nitrogen trifluoride by reacting a carbonyl fluoride with carbon tetrafluoride and binary oxides of nitrogen by reacting at temperatures of 2,000-4,000.degree. C. in an electric arc. The nitrogen trifluoride atmosphere is removed from the reactor, quenched and then cooled in a liquid nitrogen trap.
U.S. Pat. Nos. 3,235,474 and 3,356,454 disclose a technique for producing nitrogen trifluoride by the electrolysis of ammonium bifluoride. The purification processes disclosed involve, seriatim, an initial cooling of the atmosphere from the reactor, the removal of hydrogen fluoride by passing the atmosphere through a sodium fluoride trap, the removal of nitrous oxide, and some dinitrogen difluoride in a zeolite adsorber, and recovery of product nitrogen trifluoride in a liquefied nitrogen cooled trap.
Several problems have been associated with the prior art purification processes, and particularly with respect to the adsorber operation. For one reason or another, the adsorber life has been extremely short based on the estimated capacity of the adsorbent. Quite often the adsorbers had to be shut down because nitrous oxide was not being removed and because nitrogen trifluoride was being decomposed presumably because of temperature excursions in the adsorber.
The present process eliminates the basic problem of reduced adsorber life, and in addition, virtually eliminates temperature excursions within the adsorber and nitrogen trifluoride decomposition.